The accuracy of any time measurement circuit is impacted by skew and propagation delays in the path of the signal to be measured. Inaccuracies in the measurement of a signal are caused in part by differences in the rising edge and falling edge propagation delays. It is known in the arts to use calibration circuits or software-generated offsets based on mathematical predictions modeling circuit characteristics in an attempt to produce accurate time measurements. Using such techniques, accuracy of plus or minus 5 nanoseconds is sometimes achievable under certain conditions.
The problem of hardware distortion of a time signal to be measured is illustrated in FIG. 1. The period 10, or frequency of a device time signal 12 is shown. The 50% duty cycle of the device 14 is indicative of a steady time signal 12 output by the device-under-test. The automatic test equipment (ATE) receiving circuitry 16 receives the signal 12 for measurement. Due to delays introduced by the ATE receiving circuitry hardware 16, the signal 18 actually measured by the ATE measurement unit 20 is distorted. The measured signal 18 thus does not exhibit a 50% duty cycle 22. This distortion effect is anticipated, however, and is typically addressed by the inclusion of calibration hardware, or alternatively is removed from the measurement output using mathematical calibration offset techniques. The calibration hardware and offsets known in the arts are based upon assumptions concerning testing scenarios and are commonly fixed at the time of calibration.
Further problems exist with conventional time measurement circuits and methods making use of calibration circuits and measurement offsets. Typically, such calibration circuits or offset computations are based on assumptions concerning test conditions and predicted external factors such as, for example, device temperature, humidity, and power supply drift or changes. Changes in actual test conditions therefore can render the assumptions false. Failure to adapt the test equipment to actual test conditions may result in a decrease in accuracy. Adaptation to new test conditions requires that adjustments be made to the test equipment in order to compensate for the changed conditions. Also, test equipment must be recalibrated periodically in order to verify the accuracy of test results. As a result, the testing process is made undesirably complex, sensitive to external factors, and prone to error.
It would be useful and advantageous to provide improved testing circuits and methods which would decrease error and increase accuracy while reducing the influence of external factors on measurement results. Devices and methods capable of adapting to dynamic test conditions would offer further advantages including but not limited to improved accuracy under actual test conditions and extending the usefulness of available test equipment.